NTC Thermistors FAQQ[NC* Series] Why is the thermal time constant defined as 63.2% of the temperature difference?

When a thermistor kept at a certain temperature (T0) is exposed to another temperature (T1), the temperature changes exponentially, and the temperature (T) at the elapsed time (t) is:
T=(T1−T0)(1−exp(−t/τ))+T0.
If t = τ, (T−T0)/(T1−T0)=1−e−1=0.632.
The thermal time constant is defined as the time to reach 63.2% of the temperature difference.